1. Field of the Invention
The present invention relates to a surface acoustic wave resonator, a surface acoustic wave filter, a duplexer, and a communications apparatus including the same. More particularly, the present invention relates to a surface acoustic wave resonator having a piezoelectric substrate made of a single crystal material including langasite (La3Ga5SiO14).
2. Description of the Related Art
Conventionally, surface acoustic wave resonators have been used widely in band pass filters or other electronic components used in mobile communications equipment. As an example of such a surface acoustic wave resonator, a surface acoustic wave resonator or a surface acoustic wave filter having a configuration wherein an interdigital transducer (IDT) including a comb-shaped electrode is provided on a piezoelectric substrate is well known.
A piezoelectric single crystal, such as lithium niobate (LiNbO3), lithium tantalate(LiTaO3), quartz, lithium tetraborate (Li2B4O7) or the like, is used as the material for the piezoelectric substrate of such a surface acoustic wave resonator or surface acoustic wave filter.
The surface acoustic wave resonator and the surface acoustic wave filter are required to have a maximum electromechanical coupling coefficient (K2), which represents the conversion efficiency of the electricity and the mechanical energy, and a minimum temperature coefficient of group delay time (TCD), which represents the fluctuation ratio of the frequency by the temperature.
Since surface acoustic wave filters using the above-mentioned LiNbO3 or LiTaO3 material have a large K2, such surface acoustic wave filters have a large difference between the resonance frequency and the anti-resonance frequency, thereby achieving a wide band width. However, since such filters have a TCD which is larger than that of quartz, there is a problem that the operation frequency fluctuates greatly due to the temperature change.
Moreover, surface acoustic wave filters using a quartz substrate are advantageous in that they have a small frequency shift in temperature because of an extremely small TCD. However, since the quartz substrate surface acoustic wave filters have a small K2, they are disadvantageous in that they have a small difference between the resonance frequency and the anti-resonance frequency and a narrow band width.
For providing a material having a TCD smaller than that of LiNbO3 or LiTaO3, and a K2 larger than that of quartz, Li2B4O7 has been used in surface acoustic wave filters and surface acoustic wave resonators. However, since Li2B4O7 creates problems with handling and processing because of its deliquescence property and because the growth rate of the single crystal is low, use of Li2B4O7 causes very poor productivity. Moreover, although Li2B4O7 has a good temperature characteristic with respect to the frequency, it has a poor temperature characteristic with respect to K2 so that the band undesirably changes according to the temperature. Thus, there are many problems with using Li2B4O7 as a substrate material for a surface acoustic wave filter.
Recently, as a material for solving the above-mentioned problems, La3Ga5SiO14 has been proposed. La3Ga5SiO14 does not have the deliquescence property like Li2B4O7, but has a high growth rate of a single crystal compared with that of Li2B4O7. Moreover, La3Ga5SiO14 has characteristics including a TCD which is smaller than that of LiNbO3 or LiTaO3, and a K2 which is larger than that of quartz. A large number of reports have been made concerning the theoretical analyses or the experimental results relating to the prospective Euler angle or propagation direction of such an La3Ga5SiO14 single crystal substrate.
However, the analyses and measured results described in such reports relate only to alone such that the TCD is optimum. The reports and analyses are not related to and do not describe devices in which an La3Ga5SiO14 single crystal substrate is provided with other elements. Thus, the TCD is not optimum when other elements are added thereto.
To overcome the problems described above, preferred embodiments of the present invention provides a surface acoustic wave resonator which includes a langasite piezoelectric and has a greatly improved TCD.
According to one preferred embodiment of the present invention, a surface acoustic wave resonator includes a piezoelectric substrate made of a langasite single crystal, and an interdigital transducer including Al and located on the surface of the piezoelectric substrate. The Euler angle (xcfx86, xcex8, xcfx86) of the piezoelectric substrate is preferably about (0xc2x0, 140xc2x0 to 150xc2x0, 24xc2x0+1xc2x0), and the film thickness H of the interdigital transducer is preferably within the range of about 0.005 to about 0.15 with respect to the wavelength xcex of a surface acoustic wave to be excited on the piezoelectric substrate.
According to another preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate made of a langasite single crystal, and an interdigital transducer including Al and located on the surface of the piezoelectric substrate. The Euler angle (xcfx86, xcex8, xcfx86) of the piezoelectric substrate is preferably about (0xc2x0, xcex8, 24xc2x0xc2x11xc2x0), and a cut angle xcex8 of the Euler angle and a normalized film thickness H/xcex of the interdigital transducer preferably fall within an area surrounded by the straight lines linking the points represented by:
A (xcex8=140xc2x0, H/xcex=0.005)
B (xcex8=143xc2x0, H/xcex=0.005)
C (xcex8=147xc2x0, H/xcex=0.15)
D (xcex8=150xc2x0, H/xcex=0.15)
on a coordinate with respect to the cut angle xcex8 and the normalized thickness H/xcex, where H represents a film thickness of the interdigital transducer and xcex represents a wavelength to be excited on the piezoelectric.
According to still another preferred embodiment of the present invention, a method of manufacturing a surface acoustic wave device including a piezoelectric substrate made of a langasite single crystal and an interdigital transducer including Al and located on the surface of the piezoelectric substrate is provided. The method preferably includes the step of selecting a film thickness of the interdigital transducer in accordance with the Euler angle (xcfx86, xcex8, xcfx86) of the piezoelectric substrate such that fluctuation of an operation frequency of the surface acoustic wave resonator is substantially zero between about 20xc2x0 C. to about 30xc2x0 C.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.